Light of a particular wavelength falling on particles will be scattered over a range of angles, determined by the size of the particle. The size of particles can thus be inferred by measuring the scattered light over a range of angles. This principle has been used as the basis of commercially-made instruments incorporating visible laser light source for measuring particles around 0.1 μm to 3000 μm in diameter.
1. Fourier Configuration
Referring to FIG. 1, in instances where bulk size distribution is required, it is convenient to pass a representative (usually large) number of particles through a static beam of light and to detect the light scattered onto a number of photodetectors of fixed size and position. The illuminating beam is preferably at least 10 times larger (in diameter) than the largest particle.
The particles scatter light within the volume in which they intersect the beam. The finite extent of this volume means that detectors simply placed around the cell will collect light from a range of angles, reducing the capability for discrimination of different particle sizes.
Referring to FIG. 2, instruments, such as ones that employ the Fourier configuration, can use segmented photodetector arrays. These provide multiple detector elements that allow different angles of light to be resolved.
2. Telephoto (Fourier)
Measurement of larger particles requires the detection of small angles close to the focused beam. This would appear to require either reducing the size of the photodetector elements nearest the beam, or by using a weaker focusing lens in order to increase the distance of the detector plane from the lens. Referring to FIG. 3, one sophistication that can be employed, when only low angles need to be measured, is to use a telephoto lens arrangement to shorten the physical distance whilst achieving the same effective focal length.
The focusing lens has a short focal length, with a concave lens placed slightly short of its focus, to expand the scattering angles. A limitation of this system is that it has not been readily feasible to produce an expanding lens form capable of collecting larger scattering angles without severe distortion. There is a range of angles that can be measured beyond the radial extent of the expanding lens but these will be discontinuous from the low angle detection range. Large angles can not be detected with the lens arrangement as shown.
2. Binocular Fourier (Coulter)
Referring to FIG. 4, the problem of measuring large angles simultaneously with small angles was overcome by a combination of two collecting lens systems, revealed by Beckman Coulter. A Fourier lens on axis is truncated in one side to allow unobscured detection of higher angles on that side, whilst intermediate angles are detected on the opposite side of the axis. For a given minimum detector spacing, this scheme has a longer track length, since the effective focal length is the distance between the axial lens and the detector plane, and there is additional distance between the cell and the axial length to allow for the detection of higher angles.
3. Reverse Fourier
Referring to FIG. 5, the track length can be reduced using the so-called reverse Fourier configuration in which the focusing lens is in front of the sample. The effective focal length of this system is the distance between the sample and the detection plane. This allows continuous measurement from small to large forward angles. So long as they are placed in the focal plane of the lens, there is no need for any more lenses on the detectors. For large forward angle detection, it is useful to put lenses in front of individual detector elements in order to increase the light gathering area without having to use large detectors. With appropriately designed lens and aperture on these channels it possible to place these detectors closer to the sample than the focal plane. Backscatter can also be measured if the focusing lens is set a reasonable distance back from the measurement volume.